With the introduction of the FULL-Multi-Operator Core Network (FULL-MOCN) feature a common radio access network (RAN, e.g. a BSS) will be shared by multiple Mobile Switching Centres (MSCs) and/or Serving GPRS Support Nodes (SGSNs), where each MSC and/or SGSN is associated with a different Public Land Mobile Network (PLMN) identified using a unique PLMN ID value. When a Mobile Station (MS) is operating in an Evolved Universal Terrestrial Radio Access Network (E-UTRAN) service area and is attached to the UTRAN or GSM EDGE Radio Access Network (GERAN) Circuit Switched (CS) domain, a CS fallback (CSFB) from E-UTRAN access to the UTRAN/GERAN CS domain access may become necessary if the MS cannot initiate an IMS voice session because it is either not IMS registered or IMS voice services are not supported in the E-UTRAN service area. CSFB operations are described in the standards document 3GPP TS (Technical Specification) 23.272. CSFB may be performed using PS Handover to the target UTRAN/GERAN service area (i.e., PS Handover based CSFB is triggered) where the target service area supports FULL-MOCN operation.
PS Handover based CSFB can be used in a scenario wherein the serving E-UTRAN core network may select a target UTRAN/GERAN cell that is associated with a Location Area (LA) which is different from one stored in the MS. Note that a change in LA will typically be experienced for PS handover based CSFB since it involves a change in RAT (radio access technology). Consequently, the MS will initiate a Location Area Update (LAU) procedure upon arrival in the target UTRAN/GERAN cell.
As part of the LAU procedure, the MS transmits a LAU REQUEST message to the target RAN. The target RAN is responsible for forwarding the LAU REQUEST message to the correct Mobile Switching Centre (MSC) based on the PLMN ID that was selected by the serving E-UTRAN core network (during the PS Handover procedure) for use by the MS upon its arrival in the target UTRAN/GERAN service area. Thus, the MSC to which the target RAN forwards the LAU REQUEST message should be the MSC associated with the selected PLMN ID. However, this becomes problematic for PS handover based CSFB to a target cell supporting FULL-MOCN since for this scenario there will be no process for the target RAN to determine the MSC to which it should forward the LAU REQUEST message because it will not be able to associate the MS sending this message with any specific PLMN.
For the non-handover scenario where an MS is able to read system information prior to sending the LAU REQUEST message, the Skip Indicator Information Element included in this message is used to provide the target RAN with the information it needs to determine the MSC to which it is to forward the LAU REQUEST message, i.e., it contains an indication of the selected PLMN ID. Prior to sending a LAU REQUEST message, the MS has knowledge of the set of PLMNs supported by its current serving cell (indicated by system information sent in the serving cell) whenever MOCN operation is supported. Therefore, for the non-handover scenario, the MS is able to use this information to set the value of the Skip Indicator included in the LAU REQUEST message to reflect the desired PLMN ID.
However, for the case of PS Handover based CSFB described above, there is no way for the MS to determine the PLMN selected for it to use in the target cell nor the set of PLMNs supported in the target cell. The MS is therefore unable to populate the Skip Indicator with information that the target RAN needs to use to correctly forward the LAU REQUEST message. With inaccurate/invalid information present within the Skip Indicator there is a high risk that the target RAN will forward the LAU REQUEST message to a MSC that is not associated with the PLMN selected for use by the MS, which may result in the MS receiving less than optimal service. For example, the MS may be billed excessively for all CS calls made while being served by the less preferred PLMN.
The approaches described in this section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.